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First, let me explain that I had my wisdom teeth removed last Tuesday, and
between bouts of massive pain and sleep-inducing pain killers, I came across
quite a few posts on the most important dinosaurs ever to live.
Those are ornithischians for those of you who didn't understand.
Aside from the fact that I don't agree with Ken's use of para-taxa, it seems
pretty good. There are a few things I would like to point out. First is
that, as Allesandro said, Heterodontosaurs probably are the most basal
members of the Margincephalia. This was supported by Osmolska and Maryanska
in 1984 and Cooper in 1985, and Santa Luca 1983(?). George Olshevsky has
also supported this in the past.
In fact the only person who has really been opposed to this idea has been
Paul Sereno. He has explicetly demonstrated that Heterodontosaurids are
really basal ornithopods in one of his recent mega-cladogram papers. It's
based on four characters, which honestly are either coded incorrectly, or so
vaguely worded, I don't know what he's talking about. Sereno doesn't code
any of the characteristics that the "Marginoceph-boosters" cite.
Pisanosaurus has recently been shown (Sereno 1991) to be the most basal known
member of the Ornithischia, based on, among other things, the fact that it's
propubic. Before this was demonstrated however, it was widely regarded as
probably being a member of the Heterodontosauridae, based on supposed
similarities of the teeth. If I recall correctly, the teeth of Pisanosaurus
are rather badly worn, so I don't think they can be of much use in that
Agilisaurus is one of my favorite dinosaurs. It is one of two very small
ornithischians from the Lower Shaximiao (=Xiashaximiao) formation of China.
The other very small ornithischian is "Yandusaurus" multidens, which has been
put into the genus Agilisaurus by some workers.
Agilisaurus has been classified as a fabrosaurid by some, and a
hypsilophodontid by others. It's probably neither. I am not sure what it
is, but it does have two characters which may lead on to believe it's basal
to (Heterodontosauridae + Marginocephalia), those being somewhat fang-like
premaxillary teeth, and chisel-like cheek teeth.
In my opinion, this name should be abbandoned. Fabrosaurus is a nomen
dubium, and the only known 'fabrosaurid' is Lesothosaurus diagnosticus.
Plus, fabrosaurs have a very gradistic feel in the literature, as being the
ornithischians, from which all other groups sprung forth.
Maryanska and Osmolska 1984 supported an ornithischian topology that placed
Ankylosaurs as the most basal members of Ornithischia, because the most
derived members of Ankylosauria had an imperforate acetabulum, yet all other
ornithischians had perforate acetabulums. The perforate acetabulum is now
considered a synapomorphy of Dinosauria, and the condition seen in
ankylosaurs is considered a reversal by all workers.
George Olshevsky insists that the only "major" synapomorphy linking
stegosaurs to ankylosaurs is the presense of armor, which he insists is found
all over Orntihischia and Sauropoda. I'll ignore for the time being the fact
that no other ornithischians have ever been found that have any armor
preserved, as well as the fact that no sauropods, but a few very derived
titanosaurs had any sort of armor. With the sporadic disribution we see
here, it's somewhat similar to insisting that all sauropsids had beaks since
birds and turtles have beaks, and since we don'tm find them elsewhere,
they're just not preserved right, or cartilaginous.
The "minor" synapomorphies that George so readily discards include, among
other things, a broadening of the skull roof incorporating not just the
palpabral bones, but also including ossified ligaments in the form of the
medial and caudal superorbital bones. Also, there are seven premaxillary
teeth in thyreophores ancestorally, instead of the six basal to ornithischia.
I am genuinely curious about the apparently "major" synapomorphies that Geoge
has demonstrating that stegosaurs are closer to ceraopods than to
One last point. If it was up to me and I were to be making para-taxonomic
names, I would almost certainly keeep the names Hadrosauridae,
Iguanodontidae, Camptosauridae and Dryosauridae just to reduce confusion.